The invention relates to the transmission of data and power via a common transmission line to which a power source and several stations are connected, each of the stations essentially being capable of receiving power and data via the transmission line and of transmitting data via the transmission line.
If one and the same line is used for the transmission of power and of data, this line need comprise only few individual conductors or wires, no more than two conductors in the extreme case. This considerably reduces the cost of the line, especially in systems having a wide spatial distribution. This principle has accordingly been used in various forms in the past.
DE 39 07 652 A1 discloses a circuit arrangement in which a current supply unit delivers a DC voltage to a two-wire bus line to which several stations are connected. An energy storage device in the form of a capacitor is present in each individual station, is charged by the DC voltage on the bus lines via a rectifier and a resistor, and serves as the current supply to the electronic circuit of the station. For data transmission, the bus lines are intermittently short-circuited in dependence on the data to be transmitted. This means, however, that the current supply unit must deliver the DC voltage to the bus line at a comparatively high-ohmic level, so that only a limited number of stations can be supplied through this line, and in particular no high loads can be supplied at the stations.
DE 41 38 065 A1 discloses a device for the transmission of data and power via a transmission line in which said line comprises two wires and a common screen inside which the two wires are accommodated. The individual stations are connected to the two wires and to the screen, receiving half the operational current through each wire, while the full operational current is returned through the screen. The data are transmitted on both lines by means of voltage signals in counterphase. The power is derived from the two conductors by means of current sinks in the stations so as to load said voltage signals as little as possible with the power transmission. This requires comparatively expensive means for achieving a substantially constant DC voltage for supplying the circuits in the stations, and the power which can be transmitted to the stations is limited also in this case.
It is an object of the invention to provide a method as well as a system for the transmission of data and power via the same line in which the power which can be transmitted is essentially not limited, while nevertheless data can be supplied at a high-ohmic level in a very reliable transmission mode of the line.